This invention generally relates to welding systems, and is specifically concerned with a system and method for laser welding a sleeve to the inner surface of a heat exchanger tube in order to repair the tube.
Systems for laser welding sleeves to the inner surfaces of heat exchanger tubes are known in the prior art. Such systems are particularly useful in maintaining the integrity of the heat exchanger tubes used in nuclear steam generators. However, before either the utility or the limitations of such systems can be appreciated, some general background as to the structure, operation and maintenance of nuclear steam generators is necessary.
Nuclear steam generators are generally comprised of a bowl-shaped primary side, a tubesheet disposed over the top of the primary side, and a cylindrically shaped secondary side which in turn is disposed over the tubesheet. Hot, radioactive water from the reactor core circulates through the primary side of the steam generator, while non-radioactive water is introduced into the secondary side. The tubesheet hydraulically isolates but thermally connects the primary side to the secondary side by means of a number of U-shaped heat exchanger tubes whose bottom ends are mounted in the tubesheet. Hot, radioactive water from the primary side flows through the interior of these heat exchanger tubes while the exterior of these tubes comes into contact with the non-radioactive water in the secondary side in order to generate non-radioactive steam.
In the secondary side of such steam generators, the legs of the U-shaped heat exchanger tubes extend through bores present in a plurality of horizontally-oriented support plates that are vertically spaced from one another, while the ends of these tubes are mounted within bores located in the tubesheet. Small, annular spaces are present between these heat exchanger tubes and the bores in the support plates and the tubesheet which are known in the art as "crevice regions". Such crevice regions provide only a very limited flow path for the feed water that circulates throughout the secondary side of the steam generator, which causes "dry boiling", to occur wherein the feed water boils so rapidly that these regions can actually dry out for brief periods of time before they are again immersed by the surrounding feed water. This chronic drying-out causes impurities in the water to precipitate and collect in these crevice regions. These precipitates ultimately create sludge and other debris that promote the occurrence of stress corrosion cracking in the regions of the tubes surrounded by the bores of the tubesheet and the support plates which, if not repaired, will ultimately allow radioactive water from the primary side to contaminate the non-radioactive water in the secondary side of the generator.
To prevent such contamination from occurring, a repair procedure known as "sleeving" has been developed wherein a tubular sleeve formed from the same stainless steel as the damaged heat exchanger tube is slid up within the tube so that is traverses the corroded or otherwise damaged length of the tube. The ends of the sleeve are then affixed to the inner surfaces of the tubes in order to form a hydraulic "bridge" across the corroded or otherwise degraded length of the tube.
In the past, the ends of such sleeves have been affixed around the inner diameters of the heat exchanger tubes by mechanical expansions, brazing, and laser welding. While all three of these techniques have proven themselves to be effective in the field, welding is now thought to be the overall best technique for a variety of reasons. First, a properly-executed weld joint between the outer diameter or a sleeve and the inner diameter of a tube results in the strongest and most leak-proof connection between the tube and the sleeve. Secondly, welding may cause the least amount of adverse metallurgical changes to occur in the Inconel.RTM. forming the tube and the sleeve. By contrast, mechanical expansions require a significant portion of both the tube and the sleeve to be radially and inelastically deformed, thereby work-hardening the metal. Brazing necessitates the application of a large amount of heat over a large section of the sleeve and tube, which can result in adverse changes in the grain structure of the metal in these regions that renders the metal more susceptible to stress corrosion cracking. Brazing can also cause thermal stresses to occur in the tubes as a result of thermal expansion. While there are procedures which satisfactorily relieve such thermally-induced stresses, the use of such stress-relief procedures protracts the time necessary to install the sleeves which is undesirable, since for many utilities the revenue losses associated with any repair procedure exceed over a million dollars per day. Because laser welding techniques are capable of reliably creating a weld joint by the application of laser light along a very thin circle around the inner diameter of the sleeve, adverse changes in the metals forming the sleeve in the tube are minimized to very small localities in the tube. Moreover, the highly localized nature of the heat used to weld the sleeve to the tube minimizes thermal differential expansion, thereby obviating the need for time consuming stress-relief techniques. These advantages in combination with the strength, durability, and leak-proof seal that welding provides makes laser welding the most desirable choice for affixing a sleeve to the inner surfaces of damaged heat exchanger tubes.
Unfortunately, none of prior art laser welding systems that the applicant is aware of is capable of effectively and reliably welding the end of a sleeve that is less than about 0.75 inches in diameter. Applicant has observed that this limitation arises from the unavailability of either electric or pneumatic motors that have diameters smaller than about 0.75 inches but yet which are sufficiently powerful to reliably rotate the 45.degree. mirror used to deflect laser energy around the inner surface of the sleeve. The unavailability of such motors poses a significant limitation to such prior art laser welding systems, as many of the steam generators in need of sleeving operations have heat exchanger tubes whose diameters are less than 0.75 inches. While it may be possible to develop sufficiently powerful motors having smaller diameters than the ones currently available, the cost associated with such customized development would be considerable.
Clearly, what is needed is a laser welding system that is capable of welding sleeves into heat exchanger tubes that are less than 0.75 inches in diameter. Preferably, such a welding system would be inexpensive to construct, and easy and reliable in operation. Finally, it would be desirable if this system could be easily, quickly and remotely positioned within a desired tube to be sleeved by the use of robotic positioning devices already in existence.